Relativistic mass

Both of these terms are somewhat outdated. In the earlier years of relativity, it was the relativistic mass that was taken to be the "correct" notion of mass, and the invariant mass was referred to as the rest mass. Gradually, as special relativity gave way to general relativity and found application in quantum field theory, it was realized that the invariant mass was the more useful quantity and people stopped referring to the relativistic mass altogether.

When particle physicists talk about the mass of an object they always mean the rest mass. They use other terms, like energy, to refer to the relativistic mass (The reason for this is explained in the next section). The terms rest mass and relativistic mass can still be found in elementary textbooks and, especially, in popularizations of physics. There are several arguments, discussed below, as to why this terminology should be dropped. However, the fact that some relativity courses continue to use relativistic mass demonstrates that this is a matter of opinion.

Definitions

The rest mass of an object is the true invariant mass of the object. That is, all observers in inertial reference frames will agree on what the invariant mass is. Which is precisely why it is a good thing to talk about. The relativistic mass, on the other hand, is observer dependent. For an object traveling with a velocity v relative to some inertial observer, the relativistic mass is given by
:

where m is the invariant (rest) mass and c denotes the speed of light in a vacuum. This is often written as where γ (the Lorentz factor) is the quantity given by
:

Note that when the object is at rest, v = 0 and γ = 1, and the relativistic mass equals the rest mass (whence the name). At the other extreme, as the velocity approaches the speed of light, γ and the relativistic mass increase without bound.

Given an object with momentum p and energy E we can define the rest mass by the equation
:
while the relativistic mass is given by
:
Physicists usually work in units where c = 1 so that energy and relativistic mass become identical concepts. Instead of talking about relativistic mass they simply talk about energy.

Arguments regarding this terminology

The original reasoning for regarding the relativistic mass as the proper notion of mass has to do with Newton's second law of motion:
:
whereby the inertial mass of an object measures the "resistance" of that object to undergo acceleration when a given force is applied. In special relativity, this resistance becomes unbounded as v approaches the speed of light. This is another way of saying that it is impossible to accelerate anything with mass to the speed of light: one would have to push "infinitely hard".

By taking the mass to be M = γ m, with m the rest mass, it is not possible to fully retain this notion of inertia as F=ma doesn't hold in general (see below). However, the more general equation for force
:
and the equations for energy and momentum
:
:
remain valid in any reference frame when M is treated as the relativistic mass. This is particularly appealing as they are the same definitions as used in non-relativistic mechanics. If instead one uses the rest mass, the equations become:
:
:
:
In this form the velocity dependence of E and p is slightly more transparent (as long as the velocity dependence of γ is understood). In particular, it is clear that momentum is no longer a linear function of velocity.

The downside to this is that the famous Einstein equation E = m c² is only valid in the rest frame of a particle. Be that as it may, this is precisely how the equation is understood today. In a general reference frame, one should use the equation E = γ m c² or the full energy-momentum relation:
:

Some reasons for abandoning the notion relativistic mass: